1. Field of the Invention
The present invention relates to a liquid crystal display device and a method of fabricating the same. In particularl, the present invention relates to a liquid crystal display device characterized by an alignment film formed on a substrate surface, and a method of fabricating the same.
2. Description of the Related Art
Liquid crystal display devices are widely used as display devices for AV equipment and OA equipment in view of their advantages including slimness, lightness in weight, and low power consumption. Such a liquid crystal display device includes: a liquid crystal panel in which a liquid crystal is sandwiched between a pair of substrates, one of which is formed with switching devices, such as TFTs (Thin Film Transistors), arranged in a matrix fashion (hereinafter will be referred to as “TFT substrate”), the other of which is formed with a color filter (CF), black matrix (BM), and the like (hereinafter will be referred to as “CF substrate”); a backlight unit for irradiating the liquid crystal panel with backlight; and other components.
FIG. 1 is a plan view showing the configuration of a CF substrate or TFT substrate forming a liquid crystal panel related to the present invention. In fabricating the liquid crystal panel, an alignment material is applied to both of the CF substrate and the TFT substrate to form an alignment film on each substrate in an alignment film application step. Methods of forming such an alignment film include an alignment film printing method, and an ink-jet method. In both of the methods, after application of the alignment film, a provisional drying step and a subsequent alignment film main baking step are performed on the alignment film to thermally imidize the alignment film, thereby obtaining a required alignment film. In general, such an alignment film is formed so as to cover a display region extending inwardly of a seal region to be applied with sealing material 104 (see Japanese Patent Laid-Open No. 2003-222887 and No. 2006-78930 with respect to the formation of the aforementioned alignment film).
FIG. 2 shows a sectional structure of a portion extending from an edge to the display region of a liquid crystal panel related to the present invention. As shown in FIG. 2, alignment film 106 is formed on each of the opposed faces of TFT substrate 102 and CF substrate 103 so as to cover the display region. Polarizing plates 102a and 103a are bonded to the backside of the TFT substrate and the CF substrate, respectively. When bias is applied to the liquid crystal panel, residual impurity ions produced from sealing material 104 penetrate into the display region and then are adsorbed on the surface of alignment film 106 which is undesirable. Also, uncured components of sealing material 104 exude with time and then are adsorbed on the surface of alignment film 106 under application of bias to the liquid crystal panel. As a result, pixels in which contaminants, such as residual impurity ions and uncured components, are fixed, give rise to visible defects such as stain and display unevenness. Here, “display unevenness” means the disarray of liquid crystal alignment and the local color change of the panel caused by interfusion of ionic impurity into liquid crystal material.
In attempt to solve this problem, Japanese Patent Laid-Open No. 2003-222887 noted above discloses a method in which a substrate surface, in a region intermediate between sealing material and the display region, is formed with projections and depressions thereby increasing the surface area of the alignment film which adsorbs contaminants before the contaminants reach the display region.
FIG. 3 shows a sectional structure of a portion extending from an edge to the display region of the liquid crystal panel of Japanese Patent Laid-Open No. 2003-222887. As shown in FIG. 3, the effect of adsorbing contaminants can be enhanced by an increase in the surface area of alignment film 206 resulting from the formation of projections and depressions 210 on the substrate surfaces. This method, however, cannot sufficiently suppress the penetration of contaminants into the display region.
Japanese Patent Laid-Open No. 2006-78930 noted above discloses a liquid crystal panel in which there is a difference in characteristics between an alignment film formed on a TFT substrate in a region surrounded by the display region and a sealing material forming region and an alignment film formed on a CF substrate in the region surrounded by the display region and the sealing material forming region. That is, Japanese Patent Laid-Open No. 2006-78930 discloses a method in which the opposed surfaces of the alignment films are imparted with different electrostatic properties to produce a potential difference between the display region and the region surrounded by the display region and the sealing material forming region, so that impurity ions are allowed to be adsorbed in the region surrounded by the display region and the sealing material forming region.
FIG. 4 shows a sectional structure of a portion extending from an edge to the display region of the liquid crystal panel of Japanese Patent Laid-Open No. 2006-78930. As shown in FIG. 4, by producing a potential difference between alignment films 306a and 306b, a surface portion of the alignment film which is charged differently from impurity ions can perform the effect of adsorbing such impurity ions. However, adsorption is inhibited on the alignment film side which bears the same charge as the impurity ions and, hence, the impurity ions are expected to diffuse into pixels. As a result, it is possible that a long-term reliability test eventually causes pixels to give rise to visible defects such as stain and display unevenness.
In this structure, the alignment film formed in the region intermediate between the display region and sealing material has the same film structure as the alignment film formed in the display region, on at least one substrate. For this reason, residual impurity ions produced from sealing material and uncured components of sealing material penetrate into the display region easily and are finally adsorbed on the alignment film surface. This results in a problem of lowered display quality.